Conventional magnetic holders, also commonly called magnetic chucks, make use of a core member composed of a magnetically permeable or magnetically "soft" material as constituting an electromagnet with a coil wound on the core member. In these systems, the coil must be continuously energized to keep the core member magnetized by a continuous direct-current source during the time in which an object is to be held in position and the release of the object from the retention position is effected by simple disconnection of the power supply which demagnetizes the core member. During the retention period, however, the continuous passage of the energization current through the coil gives rise to the generation of heat which may cause thermal distortion of the retained object, a holding table and other surrounding components. In a machine tool, for example, such distortion is undesirable since machining accuracy is seriously influenced thereby. Furthermore, even an accidental stoppage of the power supply causes the object to be unintentionally released and this may lead to dangerous results.